Radio frequency (RF) links between a link transmitter and a link receiver may be limited in their operation due to interference received by the link receiver. This is especially true if the link receiver is located near a high power transmitter such as for RADAR.
U.S. Pat. No. 7,053,814, issued May 30, 2006, which is incorporated herein by reference, describes a photonic time-delay encoder for spreading the bandwidth of a RADAR waveform and a corresponding photonic time-delay decoder for dispreading and narrowing the bandwidth of the encoded waveform. The encoding is accomplished by a photonic encoder circuit, shown in FIG. 1A of U.S. Pat. No. 7,053,814, that selects optical delay lines for applying an equivalent pattern of RF phase shifts, with the time-pattern of phase shifts implementing a phase code. The decoding is accomplished by a photonic decoder circuit, shown in FIG. 1B of U.S. Pat. No. 7,053,814, that produces appropriately delayed taps or delayed copies of the received waveform and then applies an inverse pattern of phase shifts upon those taps. The multiple taps are then combined and directed to a photodetector. The relative delay between successive taps equals the time interval of a phase-code chip.
U.S. Pat. Nos. 7,088,886 and 7,167,614, which are incorporated herein by reference, describe an optical limiter based on seeded stimulated Brillouin scattering (SBS). This optical limiter also is described by D. Yap and W. W. Ng, in “Self-adapting limiter,” Digest 2004 International Topical Meeting on Microwave Photonics (MWP'04), pp. 193-195, IEEE (2004), which is incorporated by reference. This limiter comprises a photonic link including a laser, modulator and photodetector for which both a RF signal and a RF interferer are modulated onto the laser light, which is the optical carrier. The limiter also includes a primary SBS medium and a SBS seed generator. The optical spectrum of the light coupled to the primary SBS medium includes energy at the optical carrier and at modulation sidebands associated with the RF signal and with the RF interferer. The SBS seed generator produces light at the Stokes frequency associated with only the interferer sidebands. This Stokes seed is injected into the primary SBS medium to effectively reduce the threshold for SBS of the interferer sidebands in that primary SBS medium. However, no Stokes seed is supplied at the frequencies of the optical carrier or the signal sidebands. Thus, the power-level of the interferer sidebands is attenuated by stimulated Brillouin scattering in the seeded primary SBS medium. But the power levels of the signal sidebands and of the optical carrier are not attenuated.
The SBS seed generator includes a second SBS medium and an optical notch filter. The Stokes seeds for the RF interferer sidebands and for the optical carrier, whose power levels are above the SBS threshold in that second SBS medium, are produced in the SBS seed generator. The power level of the signal sidebands, which are weaker than the interferer sidebands, is below the SBS threshold in the second medium. Thus, no Stokes light is generated for those signal sidebands. The Stokes light for the optical carrier is removed by the notch filter. As a result, only the Stokes light associated with the interferer sidebands is injected into the primary SBS medium. B. J. Eggleton, C. G. Poulton and R. Pant in “Inducing and harnessing stimulated Brillouin scattering in photonic integrated circuits,” Advances in Optics and Photonics, 5, 536-578 (2013), which is incorporated by reference, also describe stimulated Brillouin scattering (SBS).
What is needed is an RF link to suppress the effects of narrow-bandwidth RF interference coupled to the receiver of the RF link. The embodiments of the present disclosure answer these and other needs.